JPH0652944B2 - Frequency synchronization method - Google Patents

Description

The present invention relates to a frequency synchronization system in a CATV system in which mainline transmission is transmitted by a digital encoding system.

In the conventional CATV system, it is usual to transmit the main line system and the distribution system by the analog system. In that case, a high-quality repeater for the main line, which is different from the distribution system, and a coaxial cable with a large diameter (for example, 5
C-2W) was used to maintain the transmission quality and secure the transmission distance. However, when the CATV system scale is increased, particularly when one large-scale CATV system is installed in the whole city, the analog system configures the system due to the deterioration of the characteristics (frequency, distortion, noise) accompanying the increase in the number of relays. Difficult to do.

An object of the present invention is a wave which is directly propagated in the air when converted into an analog signal (VHF band or UHF band) again when the trunk system of such a large-scale CATV system is constructed by a digital (encoding) system. It is to provide a frequency synchronization method for avoiding beat disturbance with.

The present invention further includes a PLL (phase-locked loop) circuit for receiving a broadcast wave in the VHF band or the UHF band and generating a clock synchronized with the broadcast carrier wave of the TV channel (corresponding to 0 frequency of the TV signal) to be transmitted. The transmission line clock and frame are constructed based on the frequency and sent to the digital transmission line. On the other hand, on the receiving side, the transmission clock frequency is extracted, the broadcasting carrier frequency is regenerated by the PLL circuit based on it, the desired synchronization is achieved, and the TV signal converted back to the analog signal is modulated. .

FIG. 1 shows a configuration example of the present invention. The signal in the VHF band received by the antenna ANT is the HE (head end) device 1
Is received and amplified by. Then, the down converter 2 corresponding to each TV channel (CH) demodulates into a baseband signal.

The TV signal is digitally encoded by the encoding circuit 3 (in the example of FIG. 1, intra-frame encoding (commonly called DPCM)).
Then, the bandwidth is compressed to 32 Mb / S. ), And converts it into a PCM signal (PCM tertiary group in FIG. 1) having an appropriate bit rate, and transmits it through the D / D converter 5 and the electro-optical converter 6 through the optical transmission lines OP1 to OPn.

A first point of the present invention is to create a clock synchronized with a TV signal input as a clock for digital encoding by using a PLL (phase locked loop circuit) circuit 4 prepared for each channel.

On the other hand, on the receiving side, the photoelectric converter 7 converts the optical digital signals from the optical transmission lines OP1 to OPn into electric signals. PL
The L circuit 11 extracts a clock from the output of the photoelectric converter,
In synchronization with this clock, the DPCM decoding circuit 9
The output of the D / D converter 8 is converted into a baseband signal, and the upconverter 10 remodulates it into a VHF band signal. Then, it transmits to the distribution system via the hybrid coupler 12.

The second main point of the present invention is to generate a carrier wave for up-conversion from the clock extracted on the receiving side.
This is the point of eliminating the beat obstacle with the broadcast wave propagated in the air.

Although FIG. 1 shows an example in which an optical digital transmission line is provided for each channel of a TV transmitting in the trunk line system, FIG. 2 shows an example in which they are multiplexed in the electric domain.

The signal encoded by the clock synchronized with the carrier (broadcasting) frequency for each channel is a digital multiplexer M.
It is multiplexed with UX. At that time, since the clock frequency is slightly different for each channel, so-called stuff multiplexing is required. In this case, only one optical digital line may be used as the transmission line. Also, the synchronized frequency reconstruction at the receiving side is performed by the receiving side demultiplexing device DMUX.
Then, the PLL circuit 11 may be used for each CH from the separated signals.

As described above, according to the present invention, a large city is set as one CA.
Even for a large-scale CATV system such as a TV system, it is possible to configure the trunk system by a digital system with no limitation on the transmission distance without causing a beat failure.

[Brief description of drawings]

1 and 2 show an example of the present invention, in which 1 is a headend receiver for a CATV system, 2 and 10 are down and up converters for VHF band and TV baseband, and 3 and 9 are TV signals. Transmitter and receiver devices for digital encoding, transmitter and receiver circuits for digital processing (synchronization with addition of transmission path frames) for transmission by 5, 8;
Reference numerals 6 and 7 are optical transmission / reception devices, 4 and 11 are PLLs for frequency synchronization, 4 is a PPL that produces a clock synchronized with the broadcast frequency, and 11 is a PLL that produces a carrier wave for up-conversion from the clock.

Claims (1)

[Claims] 1. A transmitting side receives a broadcast television signal, converts the broadcast television signal into a baseband band, digitally encodes it and transmits it through a digital transmission line, and broadcasts the encoded broadcast television signal on the receiving side. In the transmission method for re-converting to the wave band, the transmitting side extracts the clock from the broadcast television signal,
The encoded broadcast television signal is transmitted in synchronization with the clock, the receiving side extracts a clock from the received encoded broadcast television signal, and the broadcast encoded in synchronization with the extracted clock A frequency synchronization method characterized by reconverting television signals to the broadcast band.